refactor(change_detection): introduce enum ChangeDetectionStrategy

BREAKING CHANGE

Closes #2497

- change detection strategy type changes from string to ChangeDetectionStrategy
- CHECK_ONCE => ChangeDetectionStrategy.CheckOnce
- CHECKED => ChangeDetectionStrategy.Checked
- CHECK_ALWAYS => ChangeDetectionStrategy.CheckAlways
- DETACHED => ChangeDetectionStrategy.Detached
- ON_PUSH => ChangeDetectionStrategy.OnPush
- DEFAULT => ChangeDetectionStrategy.Default
- ON_PUSH_OBSERVE => ChangeDetectionStrategy.OnPushObserve

modules/benchmarks/src/change_detection/change_detection_benchmark.ts

@@ -15,8 +15,7 @@ import {
   ChangeDetectorGenConfig,
   BindingRecord,
   DirectiveRecord,
-  DirectiveIndex,
-  DEFAULT
+  DirectiveIndex
 } from 'angular2/src/core/change_detection/change_detection';